It is often desirable to measure one or more characteristics of a patient's respiratory gas, for example a patient under ventilation or anesthesia, or connected to an external air or oxygen supply. In particular, it is often desirable to measure carbon dioxide (CO2) levels in the respiratory gas of a patient.
Capnography monitors the concentration or partial pressure of CO2 in the respiratory gas of a patient and provides a graphic display of instantaneous CO2 concentration (FCO2) versus time or expired volume during a respiratory cycle. This display may be referred to as a CO2 waveform or capnogram. Similarly, capnometry measures and displays carbon dioxide (CO2) levels on a digital or analog monitor, for example showing the maximum inspiratory and expiratory CO2 concentrations during a respiratory cycle. Capnography (or capnometry) may be employed in a hospital setting, for example, to display CO2 levels in the respiratory gas of a ventilated patient, such as during procedural sedation. Capnography has been employed as a standard of monitoring during anesthesia for more than three decades.
Capnography is also increasingly being used by paramedics to aid in their assessment and treatment of patients in the prehospital environment. These uses include verifying and monitoring the position of an endotracheal tube. A properly positioned tube in the trachea guards the patient's airway and enables the paramedic to provide ventilation for the patient. A misplaced tube in the esophagus can lead to death. A study in the March 2005 Annals of Emergency Medicine, comparing field intubations that used continuous capnography to confirm intubations versus non-use showed zero unrecognized misplaced intubations in the monitoring group versus 23% misplaced tubes in the unmonitored group. The American Heart Association (AHA) affirmed the importance of using capnography to verify tube placement in their 2010 ACLS/CPR Guidelines.
The AHA also notes in their new guidelines that capnography, which indirectly measures cardiac output, can also be used to monitor the effectiveness of CPR and as an early indication of return of spontaneous circulation (ROSC). Studies have shown that when a person doing CPR tires, the patient's end-tidal CO2 (ETCO2), the level of carbon dioxide released at the end of expiration) falls, and then rises when a fresh rescuer takes over. Other studies have shown when a patient experiences return of spontaneous circulation, the first indication is often a sudden rise in the ETCO2 as the rush of circulation washes untransported CO2 from the tissues. Likewise, a sudden drop in ETCO2 may indicate the patient has lost their pulse and CPR may need to be initiated.
Capnography, because it provides a breath by breath measurement of a patient's ventilation, can quickly reveal a worsening trend in a patient's condition by providing paramedics with an early warning system into a patient's respiratory status. Paramedics are also now also monitoring the ETCO2 status of nonintubated patients by using a nasal cannula that collects the carbon dioxide. A high ETCO2 reading in a patient with altered mental status or severe difficulty breathing may indicate hypoventilation and a possible need for the patient to be intubated. Similarly, a low ETCO2 reading in some patients, may indicate hyperventilation.
In general, there are two types of arrangements which are employed for capnography: mainstream (non-diverting) capnography monitoring and sidestream (diverting) capnography monitoring. Sidestream, or diverting, capnography transports a portion of a patient's respiratory gas from the sampling site, through a sampling tube, to the sensor, whereas mainstream, or non-diverting, capnography does not transport gas away from the sampling site. In other words, one can view the difference between mainstream (non-diverting) capnography and sidestream (diverting) capnography as clinically measuring CO2 at the sample site versus measuring CO2 in the monitor distant from the sample site.
With mainstream monitoring, the sensor is located on a special airway adapter so that CO2 is measured directly in the patient's breathing circuit. Advantages of mainstream monitoring include faster response time, the ability to measure gas near Body Temperature and Pressure Saturated (BTPS) conditions, and operation without a water trap. However, in general mainstream monitoring has some drawbacks. Such drawbacks include the inability to monitor non-intubated patients easily.
In sidestream capnography, a sample of the patient's respiratory gas is aspirated from the breathing circuit to a sensor residing inside the monitor. In general, sidestream monitoring has some drawbacks in comparison to mainstream monitoring, including for example sample line occlusion and waveform distortions. Furthermore, the temperature of the sampled gas decreases toward room temperature during its transit from the patient connection to the monitor. This results in condensate forming on the walls of the tubing and a resulting decrease in the partial pressure of water vapor from the BTPS value to much lower values. This decrease in water vapor pressure can cause an apparent increase in CO2 concentration.
However, sidestream configurations may be used with both intubated and non-intubated patients. Accordingly, sidestream monitoring is often employed instead of mainstream monitoring, particularly in the case of non-intubated patients, .
One of the primary challenges in sidestream monitoring is separating any condensed liquid (e.g., water) from the gas sample and preventing the liquid from entering the gas monitoring device where it can damage the sensor. Most water traps available on the market today are for sampling systems with flow rates in the range of 100 ml/min or more. However with some patient groups, it is often desirable to operate with lower flow rates, for example on the order of 40-60 ml/minute.
Moreover, many water traps also require the use of a secondary flow to pull a negative pressure in the reservoir of the water trap to help separate the liquid from the gas sample.
Unfortunately, the commonly available water traps have too much dead space and volume, and the resulting impact on the gas sample characteristics is detrimental to the system performance. In particular, the gas measurement accuracy, respiratory rate range, and signal fidelity are all negatively impacted by a large dead space in a water trap in a sidestream capnography system.
Furthermore, the need for a large breath sample rate has inhibited use of sidestream monitoring in low-flow applications.
Accordingly, it would be desirable to provide a water trap which can address one or more of the issues described above.
In one aspect of the invention, a device comprises: a separation chamber and a reservoir. The separation chamber has an inlet configured to receive a gas sample from a patient; an outlet configured to output the gas sample; and an aperture disposed between the inlet and the outlet at a bottom of the separation chamber. The separation chamber defines a channel extending in a first direction between the inlet and the outlet and is configured to pass the gas sample through the channel from the inlet to the outlet. The reservoir is disposed beneath the aperture of the separation chamber. A gas permeable membrane extends across the channel such that a first portion of the membrane disposed at a top of the channel is located closer to the inlet than a second portion of the membrane disposed at a bottom of the channel.
In some embodiments, the gas permeable membrane further extends across the aperture such that the first portion of the membrane is located on a first side of the aperture closer to the inlet and the second portion of the membrane is located on a second side of the aperture opposite the first side and closer to the outlet
In some embodiments, the gas permeable membrane comprises a hydrophobic material.
In one optional variation of these embodiments, the gas permeable membrane comprises a nonwoven spunbond olefin fiber material.
In one optional variation of these embodiments, the gas permeable membrane comprises at least one of polyvinylidene fluoride and polytetrafluoroethylene.
In some embodiments, a hydrophilic material fills the aperture in the separation chamber.
In some embodiments, a material fills the aperture, wherein the material comprises at least one of polyethersulfone, mixed cellulose ester, and cellulose acetate.
In some embodiments, the reservoir is attached by threads to the separation chamber.
In some embodiments, the apparatus further comprises a measurement device connected to the outlet of the separation chamber, the measurement device being configured to measure a property of the gas sample.
In one optional variation of these embodiments, the property of the gas sample is a carbon dioxide level in the gas sample.
In another aspect of the invention, an apparatus, comprises: a tube, having an inlet, an outlet, and an aperture disposed between the inlet and the outlet at a bottom of the tube, wherein the tube defines a channel extending in a first direction between the inlet and the outlet, the channel having a cross section perpendicular to the first direction; and a reservoir disposed beneath the aperture of the tube; and a gas permeable membrane extending across the channel at an angle greater than zero degrees with respect to the cross section of the channel.
In some embodiments, the angle is between 10 degrees and 80 degrees.
In some embodiments, the gas permeable membrane comprises a hydrophobic material.
In one optional variation of these embodiments, the gas permeable membrane comprises a nonwoven spunbond olefin fiber material.
In one optional variation of these embodiments, the gas permeable membrane comprises at least one of polyvinylidene fluoride and polytetrafluoroethylene.
In some embodiments, a hydrophilic material fills the aperture in the tube.
In some embodiments, a material fills the aperture, wherein the material comprises at least one of polyethersulfone, mixed cellulose ester, and cellulose acetate.
In some embodiments, the reservoir is attached by threads to the tube.
In some embodiments, a bag is disposed within the reservoir.
In some embodiments, the reservoir includes a transparent window by which a level of liquid contained within the reservoir may be viewed from outside the reservoir.